New Haven, Conn. - The doubling of the moisture content in the stratosphere over the last 50 years was caused, at least in part, by tropical biomass burning, a Yale researcher has concluded from examining satellite weather data.

Tropical biomass burning is any burning of plant material. In the tropics this is usually the clearing of forest or grassland for agricultural purposes, mostly before the growing season.

"In the stratosphere, there has been a cooling trend that is now believed to be contributing to milder winters in parts of the northern hemisphere; the cooling is caused as much by the increased humidity as by carbon dioxide," said Steven Sherwood, assistant professor of geology and geophysics whose article appears in this month's issue of the journal, Science. "Higher humidity also helps catalyze the destruction of the ozone layer."

Cooling in the stratosphere causes changes to the jet stream that produce milder winters in North America and Europe. By contrast, harsher winters result in the Arctic.

Sherwood said that about half of the increased humidity in the stratosphere has been attributed to methane oxidation. It was not known, however, what caused the remaining added moisture.

In a study funded by the National Aeronautics and Space Administration, Sherwood examined a combination of data from a NASA satellite launched in the 1990s and operational weather satellite data archived at the Goddard Institute for Space Science in New York.

In particular, he studied monthly and yearly fluctuations of humidity in the stratosphere, relative humidity near the tropical tropopause, which is the place where air enters the stratosphere, ice crystal size in towering cumulus clouds, and aerosols associated with tropical biomass burning.

"More aerosols lead to smaller ice crystals and more water vapor entering the stratosphere," Sherwood said. "Aerosols are smoke from burning. They fluctuate seasonally and geographically. Over decades there have been increases linked to population growth."

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